The present invention relates to oil systems for gas turbine engines, and more particularly to an oil scavenge system.
Gas turbine engines employ high-speed bearings that require a continuous supply of oil for lubrication and cooling. For optimum performance, the oil flow must be properly directed into and away from the bearings. Failing to remove or scavenge oil from the bearing may be as detrimental to the bearing as insufficient oil flow because the churning of unscavenged oil within the bearing can rapidly lead to overheating.
In a conventional lubrication system, oil is supplied to the rolling elements of the bearings under pressure and then relies on gravity or its dynamics to drain back to a reservoir. One effective way to accomplish the return flow is to maintain an open, straight, and unrestricted passageway from the bearing back to the sump. This often requires that the air/oil mixture be redirected from a circumferential path within the bearing to an exit pipe, which is arranged axially or radially thereto.
To redirect the swirling bearing compartment two-phase air/oil mixture from the circumferential path direction to the axial or radial exit pipe flow direction, current oil scavenge systems use tangential scoops that transition into an integrated 90 deg bend that connects to the exit pipe. Due to minimum length requirements for the 90 deg bend, conventional scavenge ports may have an inlet plane located several degrees upstream of bottom dead center (BDC). Oil provided to the bearing compartment cavity downstream of the inlet plane needs to be carried by interfacial shear forces around the compartment to reach the inlet plane. Otherwise the oil may begin to collect in the cavity. The former typically occurs at high power settings while the latter typically occurs at low power settings such as motoring, windmilling, or idle. To permit drainage of collected oil that has not been captured by the tangential scoop, a drain is typically integrated into the tangential scoop/bend arrangement at BDC.
Although effective for particular compartment sump dimensions and moderate rotational speeds, as engine core size constraints become more aggressive and speeds increase, disadvantages of conventional scavenge port arrangements may begin to occur. In particular, as the size of the sump region decreases, the distance between the compartment seals and the free surface of the collected oil pool decreases. The reduced separation may increase the potential for oil leakage. Furthermore, interfacial shear acting on the gas/liquid interface may drive oil away from the drain at BDC. The oil may then form a recirculation zone downstream at BDC. Oil recirculation zones tend to contaminate seals, which may ultimately result in oil leakage from the compartment.
Accordingly, it is desirable to provide an oil scavenge system which efficiently directs a two-phase air/oil mixture with high circumferential flow velocity and significant velocity differences between both media into an axial or radial flow direction within compact high speed bearing compartments without oil leakage and at all operating conditions.